1. Field of the Invention
The present invention relates to a method and apparatus for measuring the water level in a well during drilling of the same. The present invention is advantageously used in drilling of geothermal or oil wells and enables easy, precise measurement of the depth of the water surface from the ground.
2. Description of the Prior Art
Drilling of geothermal, oil, and other wells is generally performed by driving a boring drill rod bearing a rotating bit at its end into the ground while injecting muddy water (slime). The muddy water is recovered from the well at the ground. Under normal conditions, i.e., when the drilling is progressing smoothly, the muddy water fills in the well and has a water level equal to the ground surface. The muddy water is necessary for smoothly drilling and conveying the crushed rock, but is a special liquid and thus is relatively expensive. Accordingly, it is very important to measure the water level of muddy water when drilling.
When the drilling bit encounters a crack in the stratum in drilling, the muddy water is absorbed through the crack and the water level suddenly descends a few hundred meters from the ground, emptying the upper part of the well. If such a crack lies before the desired stratum of geothermal vapor or oil, it is necessary to close the crack to allow the water level to be restored and thus prevent seizure of the drilling pipe by rock scraps in the well. There are two steps in this operation. First, so-called lost circulation materials (LCM), generally cottonseed, are poured into the crack from an inner pipe of the drilling pipe. Next, the crack is closed with cement provided around an outer pipe. For this operation, however, it is necessary to determine the exact position of the crack. This is found from the level of the remaining muddy water.
Conversely, if the crack lies in the desired stratum of geothermal vapor or oil, it is not desired to fill the crack with muddy water. Therefore, after such a crack is encountered, drilling is continued using relatively low cost clear water at an insufficient water level instead of muddy water at the ground level. Under such abnormal conditions, it is necessary to monitor the water level because if the water level falls too much, the water might become too hot, boil, and suddenly blow out from the well.
The conventional method for measuring the water level in a well, as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 50-144604, involves pulling up the boring drill rod, lowering a stratum detection apparatus consisting of a plurality of electrodes mounted to the ends of different wires, and detecting the current flowing between one electrode and another when dipped into the water.
During this measurement, however, it is necessary to stop the drilling. Consequently, the conventional method for measuring the water level requires much time and labor. Moreover, much valuable information is lost because of the water level changes when drilling is stopped. No method is known in the prior art for measuring the water level in the well while continuing the drilling.